Over the past few years, several satellite communication systems have been proposed. These systems are generally designed to enhance existing terrestrial cellular networks. Cellular telephone networks utilize fixed transmitting and receiving stations located in adjacent cells. Some of the satellite communication systems that will be launched later in this decade will rely on satellites operating in low Earth orbits. In cellular networks, the central transmitting and receiving antennas are stationary, and subscribers may communicate with one another while moving from cell to cell. A low Earth orbit network also allows subscribers to move, but the transmitting and receiving antennas aboard each satellite in the network are constantly in motion. Unlike some earlier communications systems which employed satellites in geosynchronous orbits, the spacecraft in low Earth orbit will move rapidly across the sky over any given place on the ground. Since the position of a low Earth orbit (LEO) satellite is not fixed with respect to a location on the ground, some LEO networks require complicated schemes for steering radio beams to users on the ground. Since each LEO satellite is only visible for a few minutes from the user's point of view, any communication through the network that lasts more than a few minutes must be handled by more than one satellite. The complex switching that is required to "hand-off" continuous communications service from one satellite to another imposes severe processing and power burdens on the LEO satellites.
One recent attempt to overcome the problem of providing a high capacity LEO communications system which ameliorates the burdens of complex switching due to frequent hand-offs between satellites is described in an allowed U.S. patent application Ser. No. 08/088,714 entitled Earth-Fixed Cell Beam Management for Satellite Communication System. This Application describes methods and apparatus which pertain to the allocation of radio beams which are generated by a constellation of satellites orbiting below geosynchronous altitude. These beams are precisely controlled so that they illuminate "Earth-fixed cells", as opposed to "satellite-fixed cells." In previous satellite communication schemes, spacecraft which are not held stationary over one particular location on the Earth in geosynchronous orbits fly over large regions of the Earth very rapidly. The radio beams generated by these fast moving spacecraft sweep across vast regions of the Earth's surface at the same rate of speed. If these beams were visible to the eye, they would paint bright circular and elliptical patches of light on the ground beneath the satellite which emitted them. In a system that employs satellite-fixed cells, the "footprint" of the radio beams propagated by the spacecraft defines the zone on the ground called a "cell" which is illuminated by the spacecraft. This satellite-fixed cell moves constantly as the spacecraft orbits around the globe.
In sharp contrast, an "Earth-fixed cell" is a stationary region mapped to an "Earth-fixed grid" that has permanent fixed boundaries, just like a city or a state. Although the rapidly moving satellites still shine their radio beams over the ground in rapidly moving footprints, the locations of the footprints at any given time do not determine the location of the unchanging Earth-fixed cells. The great advantage provided by using cells having boundaries that are fixed with respect to an Earth-fixed grid is realized when a subscriber being served by one satellite must switch to another beam in the same satellite or to a second satellite because the first is moving out of range below the local horizon. With satellite-fixed cells, this "hand-off" involves the assignment to the terminal of a new communication channel within the new beam or new satellite.
This assignment process takes time and consumes processing capacity at both the terminal and the satellite. It is also subject to blocking, call interruption, and call dropping if there is not an idle communication channel in the next serving beam or satellite. The Earth-fixed cell method avoids these problems by allocating communication channels (frequency, code, and/or time slot) on an Earth-fixed cell basis rather than on a satellite-fixed cell basis. Regardless of which satellite/beam is currently serving a particular cell, the terminal maintains the same channel assignment, thus substantially eliminating the "hand-off" problem.
The Earth-fixed cell method uses software that provides position and attitude information about each satellite in the constellation. Position data from this software enables each satellite to map the surface into an unchanging "Earth-fixed grid". Each satellite is capable of steering, transmitting and receiving beams conveying packets of information to the Earth-fixed grid. The beams are continually adjusted to compensate the effects of satellite motion, attitude changes, and the rotation of the Earth.
Unfortunately, many satellite communication and remote sensing systems that will be launched in the near future will rely on the more conventional method of utilizing satellite-fixed cells. Without the great benefits of an Earth-fixed cell method, a large portion of the microprocessor capabilities and power capacities of these systems will be diverted to internal switching tasks.
This problem of designing satellite systems which employ satellite-fixed cells but which avoid the deleterious effects of complex switching due to frequent hand-offs has presented a major challenge to the satellite business. The development of a high capacity satellite system which is capable of using satellite-fixed cells but which also minimizes hand-off overhead would constitute a major technological advance and would satisfy a long felt need within the communications and remote sensing industries.